Hepatitis C virus infection is a chronic disease that inflicts 170 million people worldwide. Infection usually results in a persistent hepatitis that can lead to cirrhosis and/or hepatocellular carcinoma. Moreover, chronic infection is associated with a number of adverse complications including lymphoma and autoimmune phenomena.Besides hepatocytes, the virus has been shown to infect peripheral blood cells. Investigators have suggested that infection of lymphocytes may be related to the development of lymphoma and autoimmune phenomena, to the progression of the chronic infection, and to maternal-fetal transmission of the virus. Although there are excellent diagnostic tests for hepatitis C virus, the currently available tests do not predict progression, maternal-fetal transmission, or immunologically related complications.We have developed an ultra-sensitive technology for the detection of molecules expressed in single cells and analyzed by flow cytometry. This technology, enzymatic amplification staining, has achieved 10-100 fold enhancements in fluorescence intensity which has been used to detect the presence of molecules that could not be detected by standard immunofluorescent methods.We propose in this grant application to assess the presence of HCV proteins and/or nucleic acids in cells infected in vitro with enzymatic amplification staining. The successful development of this capability will prompt an analysis of its utility in assessing patient samples.